1. Field of the Invention
The present invention relates to beam splitters and, in particular, to a beam splitter and/or combiner module for a wavelength division multiplex system.
2. Description of the Related Art
In optical fiber transmission systems, it is known to terminate a bidirectionally used optical fiber with what is commonly referred to as a BiDi module which provides an economical and space-conserving termination. The BiDi module, typically disposed at one end of an optical fiber, has a dichroic mirror oriented with respect to the fiber at about 45.degree. incidence and coated to reflect either a 1.3 .mu.m or 1.5 .mu.m wavelength.
U.S. Pat. No. 4,431,258 to Fye discloses an optical transmission system having a beam splitter at opposite ends of an optical fiber. Each of the beam splitters has an isosceles right triangular cross section and a multilayer dielectric coating sandwiched therebetween. Each layer of the coating is tuned for quarter-wave operation with 45.degree. incident light at 0.83 .mu.m. A first beam splitter is oriented with respect to the fiber at 43.degree. incidence and has a high nonpolarizing reflectance at a wavelength .lambda..sub.2 of 0.85 .mu.tm and a low loss transmission at a wavelength .lambda..sub.1 of 0.81 .mu.m. A second beam splitter is oriented with respect to the fiber at 47.degree. incidence and has a high nonpolarizing reflectance at the wavelength .lambda..sub.1 and a low loss transmission at the wavelength .lambda..sub.2. So arranged, the optical transmission system achieves full duplex communications at 0.81 .mu.m and 0.85 .mu.m over a single optical fiber.
U.S. Pat. No. 5,210,643 to Fujii et al. discloses a wave combining apparatus for semiconductor lasers. The combining apparatus includes a first dichroic mirror for combining two orthogonal P-polarized beams of 780 nm and 830 nm to produce a first resultant beam, and a second dichroic mirror for combining two orthogonal S-polarized beams of 780 nm and 830 nm to generate a second resultant beam. The first and second resultant beams are then combined by a polarizing beam splitter prism to form a single waveform for transmission through an optical fiber. According to Fujii et al., this wave combining apparatus is efficient because the dichroic mirrors can be selected to maximize transmission of the polarized beams.
U.S. Pat. No. 5,052,780 to Klein discloses a dichroic beam splitter for use with high power lasers. The beam splitter of Klien has a single uncoated piece of high quality optical glass which separates and rejects a fundamental beam while transmitting the harmonics therethrough.